The animate_property() method of the Animatable interface is far less
than optimal:

  • it has a direct reference to ClutterAnimation;
  • it has an interval decomposed as two values.

These issues tie the Animatable interface with the Animation object,
even though it's neither necessary nor future-proof.

Let's introduce a new method, interpolate_value(), which does not
reference ClutterAnimation and uses a ClutterInterval to express the
initial and final states.